Connective Tissue: An Introduction
Description and Embryology
Reports from clinical studies and anatomical investigation have confirmed that humans and other biological systems are structurally made in a complex interacting network of cells, tissue, and organ systems. In humans, the tissues are classified into four basic groups –muscle tissues, nervous tissues, connective tissues, and epithelial tissues. Connective tissues are considered the most abundant and diverse type of tissue in humans. They support and connect other tissues and organ systems –from the sheath of the connective tissue surrounding a bone to the blood and the tendons forming a contact point between bones and muscles. Primarily, they serve the functions of protection, support, and structural organization in humans.
All connective tissues are derived from the mesenchyme –a tissue that developed at the early stages of life from the middle layer of the embryo, the mesoderm. In itself, the mesenchyme consists of a few collagen fibers and largely of viscous ground substance. Cells in the mesenchyme are undifferentiated, spindle-shaped, and have large nuclei and fine chromatin. The specialized cells of the mesenchyme migrate from the embryo while penetrating and surrounding the developing organs. This explains why the tissues formed by the mesenchymal cells are found in complex matrixes, interspersed in the human body. In addition to forming a wide network of connective tissues, the mesenchyme also plays an important role in the formation of muscles and the vascular endothelium.
Composition of the Connective Tissues
Unlike epithelial tissues and other tissues composed of densely-packed cells, connective tissues, on the other hand, are widely dispersed within the extracellular matrix (ECM). Regardless of their position and specialized function in the body, they are typically composed of the same three components –Cells, Ground substance, and protein fibers. Combined structurally, the protein fiber and ground substance make up the extracellular matrix –structural support for surrounding cells of the body. This wide distribution network of tissues is important to the biological functions of the ECM. The composition of the ECM varies in the body, allowing for different types of tissues.
Connective tissues are made of cell types existing in both the active and the in-active forms. Cells that exist in active forms divide actively while secreting the biological component of the ground substance. In connective tissues, the most abundant cell type is the Fibroblast. The histological investigation into the functions of Fibroblast has revealed its importance in the formation of the ECM. Proteins and complex carbohydrates (Polysaccharides) secreted by Fibroblasts produce the viscous ground substances that are further embedded with fibrous proteins to form the ECM. Other cell populations include adipocytes, chondroblasts, osteoblasts, mesenchymal cells, mast cells, and macrophage cells. Prolotherapy is a treatment that helps to increase the number of these cells in damaged or weakened areas of the body.
Adipocytes store lipids, filling the cytoplasmic space. The brown adipocyte cells store lipids as droplets and have high metabolic activity. Clinical studies have suggested that the white adipocytes, in contrast, have low metabolic activity and store lipids in large drops. Chondroblasts and osteoblasts are specialized cells primarily located in bone and cartilage. These cells are basically responsible for bone formation and resorption. The mesenchymal cells of the connective are concerned with tissue healing and repair. As multipotent adult stems cells, these cells can readily differentiate into any type of connective tissue. Macrophage cell components mediate the body’s immune system and are important in defending the tissue systems against invading pathogens.
- Ground Substance
Ground substance is composed of cell adhesion proteins such as fibronectin and laminin, proteoglycans, and glycosaminoglycan, particularly hyaluronic acid. Structurally, the ground substance has been described as proteoglycan with a protein core and polysaccharide branches. It is a viscous, amorphous, and gelatinous material that primarily acts as glue compacting for every component of the ECM into a single biological mass. In addition to its compacting functions, the ground substance also allows material transfer within the ECM. Material transfer in the ECM consists of nutrient and fluid transfer aided by the high water composition of the ground substance.
There are three basic types of fibers identified in the connective tissues –elastic fibers, collagen fibers, and reticular fibers. The proportion of these fibers in tissues is a function of tissue classification. Structurally, the collagen fibers are long, straight, flexible fibers made from interlinked fibrous protein subunits. This type of fiber is widely known for its great tensile strength and characteristic resists stretching. Elastic fibers are made of the protein ‘elastin’. As the name suggests, this type of protein is stretchable and can return to its original shape after compression or stretching. They are found mostly in connective tissues of the blood vessels, spine ligaments, and skin. Reticular fibers are narrow fibers arranged in a branching network and formed from the same protein subunits as collagen fibers. Reticular fibers are widely distributed in the reticular tissues of the liver, spleen, and many other soft tissues.
There are two broad classes of connective tissues. These include proper and specialized connective tissues. Proper connective tissues include dense and loose tissue.
- Loose Connective Tissues
This type of tissue proper contains multiple cells, a loosely arranged network of fibers, and a viscous fluid matrix. The loose arrangement of fibers in this tissue accounts for its characteristic large intra-tissue spaces. Loose connective tissues also contain specialized cells called plasma cells, thick collagen fibers, and fine elastic fibers. Loose connective tissues are readily found in lymph glands, lamina propia of the alimentary, mucous membrane of the urinary and reproductive tracts, and the dermis of the skin.
- Dense Connective Tissues
As the name suggests, this category of connective tissue proper is are densely formed by a reinforced network of fiber bundles and viscous ground substance. They are elastic, protective and provides the organ system a high tensile strength. The characteristic resilience and tensile strength of these tissues make them suitable in parts of the body where impact resistance is needed. Dense connective tissues are found in the dermis of the skin, joint capsule, and the outer envelope of muscles,
- Specialized Connective Tissues
This category of tissues primarily provides structural support in many parts of the body, including the soft tissues. The bone and cartilages are perfect examples of this tissue. They are densely packed with fibers in a matrix. In bones, this matrix is strong and rigid, containing deposits of calcium salts. The lymph and blood –described as fluid connective tissue –consists of cells and fluids mediums transporting dissolved proteins, nutrients, and salts to organs requiring them.
The study of connective tissues –including their function, physiology, and pathology –are an important aspect of human medicine. Recent clinical studies investigating their role in disease conditions –such as the adipose cells in arteriosclerosis and specialized connective tissue in bone disease –have further prompted interest in understanding these tissues.